After this CL, you can use <sky-element> to describe custom elements. The
current iteration is very basic and is hardcoded to "example", but its a start.

This CL renames the |init| function to |_init| to prevent importers from
calling it directly. Also, we now pass the <script> element to |_init| to give
some context.

R=ojan@chromium.org, eseidel@chromium.org

Review URL: https://codereview.chromium.org/950493003